A student kneels today at candles placed in Haltern, Germany. Tuesday's crash of a German jetliner claimed 150 lives, including 16 children from a German school in Haltern and their two teachers who were returning from an exchange trip to Spain.

SEYNE-LES-ALPES, France — French investigators cracked open the badly damaged black box of a German jetliner today and sealed off the rugged Alpine crash site where 150 people died when their plane slammed into a mountain.

The cockpit voice recorder was being mined by investigators for clues into what sent the Germanwings Airbus 320 into a midflight dive Tuesday after pilots lost radio contact over the southern French Alps during a routine flight from Barcelona to Duesseldorf.

Helicopters surveying the scattered debris lifted off at daybreak, and crews traveled slowly over land to the remote crash site through fresh snow and rain, threading their way to the craggy ravine. Bereaved families and the French, German and Spanish leaders were expected later today.

“The black box is damaged and must be reconstituted in the coming hours in order to be usable,” French Interior Minister Bernard Cazeneuve said.

Key to the investigation is what happened the two minutes of 10:30 a.m. and 10:31 a.m., said Segolene Royal, a top government minister whose portfolio includes transport. From then on, air traffic controllers were unable to make contact with the plane.

The voice recorder takes audio feeds from four microphones within the cockpit and records all the conversations between the pilots, air traffic controllers as well as any noises in the cockpit. The flight data recorder, which Cazeneuve said had not been retrieved yet, captures 25 hours’ worth of information on the position and condition of almost every major part in a plane.

Royal and Cazeneuve both emphasized that terrorism is considered unlikely in the crash.

Investigators retrieving data from the recorder will focus first “on the human voices, the conversations” followed by the cockpit sounds, Transport Secretary Alain Vidalies told Europe 1 radio. He said the government planned to release information gleaned from the black box as soon as it can be verified.

Germanwings said 144 passengers and six crew members were on board the flight. The victims included two babies, two opera singers, an Australian mother and her adult son vacationing together, and 16 German high school students and their two teachers returning from an exchange trip to Spain.

“Nothing will be the way it was at our school anymore,” said Ulrich Wessel, the principal of Joseph Koenig High School in the German town of Haltern.

The plane, operated by Germanwings, a budget subsidiary of Lufthansa, was less than an hour from landing in Duesseldorf when it unexpectedly went into a rapid eight-minute descent. The pilots sent out no distress call.